11 Answers

The individual light receptors (rods) in the eye can respond to a single photon, but that reception won’t trigger a signal to the brain, so it can’t really be said that that photon is “seen”. For a signal to be sent, at least between five to nine photons must reach the rods within a time period of one hundred milliseconds. (source)

well…. one of my physics teachers argued that you could. as a homework assignment, he had everyone go home find the darkest room possible, and to make sure no light was coming in and then wait until you see something. Not many people actually did it, but you were supposed to be able to see spots or small flashes of light after about 15 minutes when your were adjusted as much as they could. those spots/flashes he said were photons.

No. We can see a general region where a single photon may or may not have existed immediately before we saw it. The mere act of looking for a photon changes its placement. At least, that’s according to my understanding of quantum physics.

An old Psych professor once said that the cells lining your retina were thought to be sensitive enough to detect a single photon.
That said, you probably wouldn’t be able to get anything coherent out of it.